Method of atomic layer etching using hydrogen plasma

ABSTRACT

A method for etching a target layer on a substrate by a dry etching process includes at least one etching cycle, wherein an etching cycle includes: depositing a carbon halide film using reactive species on the target layer on the substrate; and etching the carbon halide film using a plasma of a non-halogen hydrogen-containing etching gas, which plasma alone does not substantially etch the target layer, thereby generating a hydrogen halide as etchant species at a boundary region of the carbon halide film and the target layer, thereby etching a portion of the target layer in the boundary region.

CROSS-REFERENCE TO RELATED APPLICATION

This Application claims priority to U.S. Provisional Patent Application No. 62/512,991, entitled “METHOD OF ATOMIC LAYER ETCHING USING HYDROGEN PLASMA,” filed May 31, 2017, the disclosure of which is hereby incorporated herein by reference.

BACKGROUND

Field of the Invention

The present invention generally relates to a method of cyclic dry etching of a layer constituted by silicon or metal oxide, for example.

Description of the Related Art

Atomic layer etching (ALE) is cyclic, atomic layer-level etching using an etchant gas adsorbed on a target film and reacted with excited reaction species, as disclosed in Japanese Patent Laid-open Publication No. 2013-235912 and No. 2014-522104. As compared with conventional etching technology, ALE can perform precise, atomic layer-level continuous etching on a sub-nanometer order to form fine, narrow convex-concave patterns and may be suitable for, e.g., double-patterning processes. As an etchant gas, Cl₂, HCl, CHF₃, CH₂F₂, CH₃F, H₂, BCL₃, SiCl₄, Br₂, HBr, NF₃, CF₄, C₂F₆, C₄F₈, SF₆, O₂, SO₂, COS, etc. are known. However, it is revealed that in-plane uniformity of etching of a film on a substrate by ALE is not satisfactory when etching an oxide mineral film such as silicon oxide film.

When etching Si or GaAs by ALE using Cl₂ as an etchant gas, relatively good in-plane uniformity of etching can be obtained. However, when etching a silicon oxide film by ALE using a fluorocarbon such as C₄F₈ as an etchant gas, good in-plane uniformity of etching is not obtained. This is because the etchant gas is adsorbed on a surface of a substrate through physical adsorption, not chemical adsorption, despite the fact that conventionally, the adsorption of an etchant gas is sometimes called “chemisorption.” That is, conventional ALE etches a metal or silicon oxide film by etchant gas physically adsorbed on its surface, wherein the adsorbed etchant gas reacts with excited species, and also by etchant gas which remains in the reaction space after being purged, causing gas-phase etching. As a result, in-plane uniformity of etching suffers. If an etchant gas is chemisorbed on a surface of a substrate, the adsorption is “chemisorption” which is chemical saturation adsorption which is a self-limiting adsorption reaction process, wherein the amount of deposited etchant gas molecules is determined by the number of reactive surface sites and is independent of the precursor exposure after saturation, and a supply of the etchant gas is such that the reactive surface sites are saturated thereby per cycle (i.e., the etchant gas adsorbed on a surface per cycle has a one-molecule thickness on principle). When chemisorption of an etchant gas on a substrate surface occurs, high in-plane uniformity of etching can be achieved. Conventional ALE, even though it calls adsorption “chemisorption,” in fact adsorbs an etchant gas on a substrate surface (e.g., SiO₂ and SiN) by physical adsorption. If adsorption of an etchant gas is chemisorption, in-plane uniformity of etching should logically be high and also the etch rate per cycle should not be affected by the flow rate of the etchant gas or the duration of a pulse of etchant gas flow after the surface is saturated by etchant gas molecules. However, none of conventional etchant gases satisfies the above.

The above and any other discussion of problems and solutions in relation to the related art has been included in this disclosure solely for the purpose of providing a context for the present invention, and should not be taken as an admission that any or all of the discussion was known at the time the invention was made.

SUMMARY

In some embodiments, a film constituted by components of an etchant (which may be referred to as “an etchant film”) is deposited on a surface of a target layer, and then, the etchant film as well as the target layer are etched using plasma treatment. By conducting the deposition step and the etching step and repeating them alternately as necessary, the target layer can be etched by a substantially constant predetermined quantity at each time of conducting the deposition step and the etching step as an etching cycle. In the above, the etchant components do not serve initially as an etchant gas which etches the target layer, but form a film on the surface of the target layer. In this disclosure, a “film” refers to a layer continuously extending in a direction perpendicular to a thickness direction substantially without pinholes to cover an entire target or concerned surface of the target, and typically a film is formed through reaction using reactive species, rather than simply formed by chemical or physical adsorption of gas molecules on the surface; thus, the film can grow in a thickness direction beyond an atomic layer thickness as a deposition process continues. Further, “depositing a film” is not merely providing reactive sites on a target layer surface, but is physically depositing a film having a certain thickness.

In some embodiments, when the etchant film deposited on the surface of the target layer is etched by reactive ions such as hydrogen plasma, the components of the etchant film are dissociated and reacted with the reactive ions, generating reactive etchant species which can etch a portion of the target layer at a boundary between the etchant film and the target layer. In the above, only a certain thickness of the etchant film at the boundary can contribute to etching reaction of the target layer because the reactive etchant species need to be generated in a vicinity of the boundary. Further, since the reactive ions which etch the etchant film do not etch the target layer, when the etchant film is removed by the reactive ions, the etching reaction of the target layer stops. Thus, by the above method, a constant amount of the target layer can always be etched by one etching cycle, and thus, controllability and operability of the etching processes are high.

For example, when the target layer is constituted by SiO₂, a fluorocarbon film (CF film) is deposited as an etchant film on a surface of the target layer by plasma-enhanced CVD or thermal CVD, followed by exposing the etchant film to a hydrogen plasma, so as to remove the etchant film and simultaneously etch the surface of the target layer. In the above, “simultaneously” refers to occurring substantially or predominantly at the same time or substantially or predominantly overlapping timewise. In the above, the depth of the etched portion of the target layer increases as the thickness of the etchant film increases; however, the depth of the etched portion reaches a plateau and no longer increases when the thickness of the etchant film reaches a certain value. Similarly, the depth of the etched portion of the target layer increases as the duration of exposure of the etchant film to the plasma increases; however, the depth of the etched portion reaches a plateau and no longer increases when the duration of exposure of the etchant film to the plasma reaches a certain value. That is, in some embodiments, the above-discussed etching process is a self-limiting reaction process with the two parameters having saturation points.

In the above, the CF film is removed by the hydrogen plasma as gases such as CH₄, HF, etc., and in a region in the vicinity of a surface of the SiO₂ layer, a portion of the SiO₂ layer also is simultaneously removed as gases such as SiF₄, H₂O, etc. Only a portion of the CF film near the boundary contributes to removal of the portion of the SiO₂ layer, and the remaining portion of the CF film does not contribute to removal of the SiO₂ layer, but is simply removed by the hydrogen plasma. Thus, the depth of the etched portion of the SiO₂ layer reaches a plateau in relation to the thickness of the CF film. Also, since etching of the SiO₂ layer using the hydrogen plasma is effective only when the CF film exists, when the CF film is removed (used up), the etching of the SiO₂ layer stops, i.e., the depth of the etched portion of the SiO₂ layer reaches a plateau also in relation to the duration of the hydrogen plasma exposure.

In some embodiments, the target layer can be constituted by any material (e.g., SiO₂, TiO₂) which can be etched using a hydrogen halide derived from a CF film. The above etching has high selectivity as to the material constituting the target layer, and thus, for example, a film constituted by silicon or metal oxide can be selectively etched without substantially etching a film constituted by silicon or metal nitride or carbide. In some embodiments, the target layer can be constituted by a material which can be etched using a halogen other than fluorine, as long as a suitable etchant film is selected. Typically, the etchant film is exposed to an oxygen plasma in a reactive ion etching (RIE) process. Since the etching process involves a self-limiting reaction process (or saturation process), high controllability can be realized.

For purposes of summarizing aspects of the invention and the advantages achieved over the related art, certain objects and advantages of the invention are described in this disclosure. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.

Further aspects, features and advantages of this invention will become apparent from the detailed description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of this invention will now be described with reference to the drawings of preferred embodiments which are intended to illustrate and not to limit the invention. The drawings are greatly simplified for illustrative purposes and are not necessarily to scale.

FIG. 1 is a schematic representation of a plasma-assisted cyclic etching apparatus usable in an embodiment of the present invention.

FIG. 2 shows a schematic process sequence of plasma-assisted cyclic etching in one cycle according to an embodiment of the present invention wherein a step illustrated in the sequence represents an ON state whereas no step illustrated in the sequence represents an OFF state, and the length of each ON and OFF states does not represent duration of each process.

FIG. 3 illustrates schematic drawings of one cycle of an etching process according to an embodiment of the present invention, wherein (a) represents a deposition step, (b) represents a pre-etching step, and (c) represents an etching step.

FIG. 4 is a flow chart showing an atomic layer etching (ALE) according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In this disclosure, “gas” may include vaporized solid and/or liquid and may be constituted by a single gas or a mixture of gases. In this disclosure, a process gas introduced to a reaction chamber for deposition through a showerhead may be comprised of, consist essentially of, or consist of an etchant gas and an additive gas. The additive gas typically includes a dilution gas for diluting the etchant gas and reacting with the etchant gas when in an excited state. The etchant gas can be introduced with a carrier gas such as a noble gas. Also, a gas other than the process gas, i.e., a gas introduced without passing through the showerhead, may be used for, e.g., sealing the reaction space, which includes a seal gas such as a noble gas. In some embodiments, the term “etchant gas” refers generally to at least one gaseous or vaporized compound that participates in etching reaction that etches a target layer on a substrate, and particularly to at least one compound that deposits on the target layer in an excited state and etches the target layer when being activated by a plasma. The term “reactant gas” refers to at least one gaseous or vaporized compound that contributes to deposition of the etchant film, activation of the etchant film, or catalyzes an etching reaction by components of the etchant film. The reactant gas can serve as a purging gas. The dilution gas and/or carrier gas can serve as “reactant gas”. The term “carrier gas” refers to an inert or inactive gas in a non-excited state which carries an etchant gas to the reaction space in a mixed state and enters the reaction space as a mixed gas including the etchant gas.

Further, in this disclosure, any two numbers of a variable can constitute a workable range of the variable as the workable range can be determined based on routine work, and any ranges indicated may include or exclude the endpoints. Additionally, any values of variables indicated (regardless of whether they are indicated with “about” or not) may refer to precise values or approximate values and include equivalents, and may refer to average, median, representative, majority, etc. in some embodiments. Additionally, the terms “constituted by” and “having” refer independently to “typically or broadly comprising”, “comprising”, “consisting essentially of”, or “consisting of” in some embodiments. Further, an article “a” or “an” refers to a species or a genus including multiple species. In this disclosure, any defined meanings do not necessarily exclude ordinary and customary meanings in some embodiments.

In the present disclosure where conditions and/or structures are not specified, the skilled artisan in the art can readily provide such conditions and/or structures, in view of the present disclosure, as a matter of routine experimentation. In all of the disclosed embodiments, any element used in an embodiment can be replaced with any elements equivalent thereto, including those explicitly, necessarily, or inherently disclosed herein, for the intended purposes. Further, the present invention can equally be applied to apparatuses and methods.

The embodiments will be explained with respect to preferred embodiments. However, the present invention is not limited to the preferred embodiments.

Some embodiments provide a method for etching a target layer on a substrate by a dry etching process which comprises at least one etching cycle, wherein an etching cycle comprises: (i) depositing a carbon halide film using reactive species on the target layer on the substrate, wherein the carbon halide film and the target layer are in contact with each other; and (ii) (1) etching the carbon halide film using a plasma of a non-halogen hydrogen-containing etching gas without etching the target layer, which plasma alone does not substantially etch the target layer, and thereby (2) generating a hydrogen halide as etchant species at a boundary region of the carbon halide film and the target layer, thereby etching a portion of the target layer in the boundary region. The “reactive species” in step (i) do not substantially etch the target layer but form an etchant film on the target layer. In the disclosure, “substantially zero” or the like (e.g., “not substantially etch”) may refer to an immaterial quantity, less than a detectable quantity, a quantity that does not materially affect the target or intended properties, or a quantity recognized by a skilled artisan as nearly zero, such as that less than 10%, less than 5%, less than 1%, or any ranges thereof relative to the total or the referenced value in some embodiments.

In some embodiments, the target layer is constituted by silicon or metal oxide, wherein the metal may be Ti, W, Ta, etc., such as SiO₂, metal oxide (e.g., TiO₂), or a metal alone which is etchable by a hydrogen halide, etc. When an etchant gas contains Cl or Br, the target layer may be constituted by Al₂O₃, AlN, GaAs, GaN, GaP, InP, etc. In some embodiments, the target layer may be constituted by polyvinyl chloride when an etchant gas contains Cl. A skilled artisan can determine a possible combination of an etchant gas and a target layer, based on routine experimentation as necessary. In some embodiments, the target layer is formed in trenches or vias including side walls and bottom surfaces, and/or flat surfaces, by plasma-enhanced CVD, thermal CVD, cyclic CVD, plasma-enhanced ALD, thermal ALD, radical-enhanced ALD, or any other thin film deposition methods. Typically, the thickness of the target layer is in a range of about 50 nm to about 500 nm (a desired film thickness can be selected as deemed appropriate according to the application and purpose of film, etc.).

In some embodiments, in step (i), the carbon halide film is deposited by a gas phase reaction wherein the reactive species are those of an etchant gas or gases constituted by a halogen and a carbon. In some embodiments, the halogen is F, Cl, or Br. In some embodiments, any suitable etchant gases including conventional etchant gases (e.g., discussed in the section of “Related Art”) can be used. Since the etchant gas in step (i) does not serve as a reactive etching gas which etches directly the target layer, but serves as a gas for deposition, preferably, the etchant gas is CxFy having a double or triple bond wherein x and y are integers and x is at least 2, e.g., C₂F₂, C₂F₄, C₃F₆, C₄F₈, C₅F₈, C₅F₁₀, or any combination of the foregoing. These gases tend to readily form a fluoropolymer in an excited state. In step (i), the reactant gas is selected in order to deposit an etchant film, rather than etching the target layer, e.g., no oxygen-containing gas is used since an oxygen plasma generates active etching species from the etchant gas for etching a silicon oxide film or the like. In some embodiments, the reactant gas is a noble gas such as Ar and He. In some embodiments, by selecting a suitable reactant gas and other deposition conditions, an etchant gas that is usually used for etching an silicon oxide film, such as CF₄, C₂F₆, C₃F₈, C₄F₁₀, etc. can be used.

In some embodiments, an etchant gas other than that containing fluorine may be used for depositing an etchant film in step (i). For example, an alkyl halide such as C₂H₃Cl can be used. Further, SF₆, HCl, HBr, etc. can be used in combination with a hydrocarbon such as CH₄.

In some embodiments, an etchant film may be deposited by a surface reaction such as atomic layer deposition (ALD), wherein the etchant gas chemisorbs onto a surface of the target layer, followed by exposing the surface to reactive species of a reactant gas.

In some embodiments, step (i) uses a gas phase reaction which is plasma-enhanced CVD. In some embodiments, the plasma-enhanced CVD comprises: (a) continuously feeding a noble gas to a reaction space wherein the substrate is placed; (b) continuously feeding a carbon halide gas to the reaction space; and (c) after elapse of a preset duration of steps (a) and (b) without excitation of the noble gas and the carbon halide gas, applying RF power to the reaction space to deposit the carbon halide film on the target layer, wherein no oxidizing gas is fed to the reaction space throughout steps (i) to (iii). In the above, the term “continuously” refers to without interruption in space (e.g., uninterrupted supply over the substrate), without interruption in flow (e.g., uninterrupted inflow), and/or at a constant rate (the term need not satisfy all of the foregoing simultaneously), depending on the embodiment. In some embodiments, “continuous” flow has a constant flow rate (alternatively, even though the flow is “continuous”, its flow rate may be changed with time). In some embodiments, a duration of step (c) is shorter than the preset duration of steps (a) and (b). Since RF power is applied to the reaction space for a short time for deposition, it is important to fill the reaction space fully with the carbon halide gas before applying RF power.

In some embodiments, step (i) continues until a thickness of the carbon halide film falls within a range of 0.5 nm to 10 nm, preferably 1 nm to 5 nm, which is near a plateau thickness (or saturation thickness), beyond which an etched quantity of the target layer in step (ii) does not increase even if the thickness of the carbon halide film further increases.

In some embodiments, a duration of step (i) is correlated with a thickness of the etched portion of the target layer until the thickness of the etched portion of the target layer reaches a plateau while the duration of step (i) increases, and step (i) continues until the thickness of the etched portion of the target layer reaches the plateau or a point near the plateau. The mechanisms of the above are discussed earlier in this disclosure, although the mechanisms are not intended to limit the invention.

In some embodiments, step (ii) continues until the carbon halide film is substantially entirely etched, indicating that substantially the entire portion of the boundary region of the target layer is etched. In the above, “substantially the entirety” or the like may refer to the entirety short by an immaterial quantity, by a detectable quantity, by a quantity that does not materially affect the target or intended properties, or by a quantity recognized by a skilled artisan as an insignificant value, such as that less than 10%, less than 5%, less than 1%, or any ranges thereof relative to the total or the referenced value in some embodiments. Preferably, step (ii) continues until the carbon halide film is completely etched, indicating that the entire portion of the boundary region of the target layer is completely removed. The “boundary region” of the target layer is defined as a region which is etched when the carbon halide film is completely etched. When a residue of the carbon halide film remains on the surface of the target layer, the residue may at least partially interfere with etching of the target layer, affecting in-plane uniformity of etched depth of the target layer.

In some embodiments, in step (ii), a thickness of the etched portion of the target layer is 0.1 nm to 2.0 nm, preferably 0.5 nm to 1.0 nm, which is thicker than a thickness of a monolayer defined in atomic layer etching (ALE) which is less than 0.1 nm/cycle.

In some embodiments, in step (ii), the non-halogen hydrogen-containing etching gas is hydrogen or ammonia or a mixture of hydrogen or ammonia and a rare gas. In some embodiments, the hydrogen-containing etching gas contains no oxygen. However, any suitable reactant gas can be selected to activate the etchant film for etching the target layer by reactive ion etching. In some embodiments, a noble such as Ar and He gas, or nitrogen gas may be used as a reactant gas in combination with a hydrogen-containing gas.

In some embodiments, in step (ii), the carbon halide film is etched by reactive ion etching (RIE). The RIE may be inductively-coupled plasma etching or capacitively-coupled plasma etching. In some embodiments, the capacitively-coupled plasma etching comprises: (a) continuously feeding a reactant gas to a reaction space wherein the substrate is placed; and (b) after elapse of a preset duration of step (a) without excitation of the reactant gas, applying RF power to the reaction space to etch the carbon halide film and the target layer.

In some embodiments, the etching cycle comprised of steps (i) and (ii) is repeated at least two times until a desired etched depth of the target layer is obtained. Since the etching cycle is a self-limiting etching process or saturation process, the etched depth of the target layer is proportional to the number of cycles performed.

In some embodiments, step (i) and step (ii) are continuously conducted in the same reaction chamber. In the above, the word “continuously” refers to at least one of the following: without breaking a vacuum, without being exposed to air, without opening a chamber, as an in-situ process, without interruption as a step in sequence, without changing process conditions, and without causing chemical changes on a substrate surface between steps, depending on the embodiment. In some embodiments, an auxiliary step such as purging or other negligible step in the context does not count as a step, and thus, the word “continuously” does not exclude being intervened with the auxiliary step.

Some embodiments are explained with reference to the drawings, but are not intended to limit the invention.

FIG. 3 illustrates schematic drawings of one cycle of an etching process according to an embodiment of the present invention, wherein (a) represents a deposition step, (b) represents a pre-etching step, and (c) represents an etching step. Prior to the deposition step (step (a)), a substrate 41 (e.g., a silicon wafer or other semiconductor wafer), on which a target layer 42 is formed, is provided in a reaction space. In the deposition step, a carbon halide film (an etchant film) 43 is deposited on the target layer 42 at a thickness no less than a saturation thickness T1, wherein the carbon halide film 43 and the target layer 42 are in contact with each other. The saturation thickness T1 is defined as a thickness, a portion of the etchant film beyond which does not contribute to etching of the target layer, i.e., an etched quantity of the target layer does not increase even if the thickness of the etchant film further increases beyond the saturation thickness T1. In the pre-etching step (step (b)), the etchant film 43 is exposed to a plasma such as a hydrogen plasma (hydrogen radicals H*), thereby etching the etchant film, generating gaseous components such as hydrocarbon gas and hydrogen halide, etc. therefrom, and removing the top portion of the etchant film 43 above the saturation thickness T1 through the following chemical reaction, for example (where the etchant film is constituted by CxFy wherein x and y are integers): CxFy+(4x+y)H*→xCH₄ +yHF  (1)

The above reaction is a general reaction and simplified since the composition of fluorocarbon varies depending on the deposition method, the type of etchant gas, etc. Further, although hydrocarbons other than CH₄ may be generated, such hydrocarbons are likely to be eventually decomposed to CH₄, and thus, the reaction can be represented by CH₄. Through reaction (1), by the hydrogen radicals, carbon components can be removed from the etchant film, while generating HF components, which in turn etch the SiO₂ film.

When the etching of the etchant film 43 progresses and the etchant film 43 is etched to the saturation thickness T1, gaseous components such as HF, etc. from the etchant film 43 simultaneously generate gaseous components such as SiF₄, 2H₂O, etc. from a portion of the target layer 42 having a depth T2 by using the HF gas at a boundary region 44 through the following chemical reaction, for example: SiO₂+4HF→SiF₄+2H₂O  (2)

Through reaction (2), the SiO₂ film is etched wherein generated H₂O can further assist the etching reaction. This etching is chemical etching, and thus, etching selectivity can remarkably be high, e.g., between SiO₂ and SiN (high etch rate against SiO₂, but substantially no etch rate against SiN), and also, conformal etching (uniform etching) can be performed. These features cannot be achieved by conventional etching methods.

The boundary region 44 is comprised of a boundary region of the etchant film 43 having the saturation thickness T1 and a boundary region of the target layer 42 having a depth T2. The total thickness of the boundary region (T1+T2) may depend on the ion energy in plasma, e.g., depending on RF power and the pressure of the reaction space. The boundary region 44 may be composed of an intermediate constituted by mixed components such as SiCOF. As shown in reactions (1) and (2), the pre-etching step (step (b)) is comprised of two steps, wherein the first step is etching the etchant film (e.g., carbon halide film) and generating etchant species (e.g., hydrogen halide) by hydrogen radicals, and the second step is etching the target layer by the etchant species. The second step can be performed without a plasma (without applying RF power), whereas the first step uses a plasma. However, the first and second steps overlap each other, i.e., the second step of etching the target layer by the etchant species starts before the end of the first step.

In the etching step (step (c)), the boundary region or intermediate layer 44 is removed as gaseous components, wherein the target layer 42 is etched by the depth T2 to obtain an etched target layer 45. It should be noted that although steps (b) and (c) are separately shown for an easy understanding of the principle of the steps, these steps rather concurrently occur. Since the plasma alone does not substantially etch the target layer, the etching of the target layer 45 stops when the boundary region 44 is removed.

In some embodiments, the process may be performed as illustrated in FIG. 4 which shows a flow chart of an atomic layer etching (ALE). In step a, a CF film is deposited by plasma-enhanced or thermal CVD or any suitable method which can preferably perform conformal deposition (e.g., hexafluoropropylene oxide (HFPO) thermal deposition). After step b (purging the reaction space), in step c, the CF film is exposed to a hydrogen plasma, thereby generating HF at a surface of the target layer (e.g., SiO₂). In this process, the presence of hydrogen radicals may be sufficient or additionally, ion bombardment may be necessary in some embodiments. After step c, the reaction space is purged in step d. Steps a to d constitute one cycle of ALE, which may be repeated multiple times until a desired thickness of the target layer is etched. Since the etchant film (CF film) is exposed to a hydrogen plasma and etched, and the target layer is not exposed to the hydrogen plasma, less or substantially no plasma damage is caused to the target layer.

In some embodiments, the process sequence may be set as illustrated in FIG. 2. FIG. 2 shows a schematic process sequence of plasma-assisted cyclic etching in one cycle according to an embodiment of the present invention wherein a step illustrated in the sequence represents an ON state whereas no step illustrated in the sequence represents an OFF state, and the length of each ON and OFF states does not represent duration of each process. In this embodiment, one etching cycle comprises a deposition step (plasma-enhanced CVD in this embodiment) and an etching step. The deposition step comprises “Step 1”, and the etching step comprises “Purge 1”, “Step 2”, “Step 3”, and “Purge 2”.

In “Step 1” of the deposition step which is a plasma-enhanced CVD step, a dilution gas such as Ar and/or He is continuously fed to a reaction space wherein the substrate is placed, while a carbon halide gas is continuously fed to the reaction space, while continuously applying RF power to the reaction space without feeding a hydrogen gas. A halogen-containing film (e.g., a carbon halide film) having a desired thickness (e.g., 0.5 nm to 10 nm, typically about 2 nm) is deposited as an etchant film on a target layer in “Step 1” of the deposition step. Step 1 is a step of deposition of the etchant film (extending in a direction perpendicular to a thickness direction without pinholes and entirely covering a concerned area of the target layer), which step is more than a step of providing reactive sites on the target layer. The deposition step may be any suitable deposition step including those of thermal CVD, PVD, PEALD, etc., and a skilled artisan in the art can readily modify “Step 1” accordingly.

After “Step 1”, “Purge 1” of the etching step starts, wherein a hydrogen gas (etching gas) is continuously fed to the reaction space without feeding the dilution gas and the carbon halide gas and without applying RF power. The etching gas may be a gas containing hydrogen such as ammonia or a mixture of a hydrogen-containing gas and a rare gas (e.g., Ar, He). “Purge 1” is for removing non-reacted products, unwanted by-products, and any remaining deposition gas(es) and for stabilizing the reaction space and getting ready for etching. After elapse of a preset duration of “Purge 1”, in “Step 2”, RF power is applied to the reaction space to etch the halogen-containing film (etchant film, e.g., carbon halide film) and the target layer while feeding the hydrogen gas (etching gas) without feeding the halogen-containing gas to generate hydrogen radicals which in turn generates etchant species such as a hydrogen halide as a result of reaction with the etchant film such as carbon halide. The etchant species such as a hydrogen halide chemically etches the target layer surface in contact with the etchant species at the boundary region between the target layer and the halogen-containing film. In “Step 2”, the chemical etching of the target layer begins and progresses as the etchant species such as a hydrogen halide is generated, and in “Step 3” where the hydrogen gas flow and the application of RF power are stopped, since some of the etchant species remains, the residual etchant species continues etching the target layer surface until the residual etchant species is consumed or “Purge 2” begins. Any suitable means for generating hydrogen radicals can be used in “Stet 2”. The duration of “Step 2” can be determined according to the thickness of the halogen-containing film, etc. In “Step 2”, the halogen-containing film is etched or removed in its entirety. The duration of “Step 3” can be determined according to how much residual hydrogen halide is left at the end of “Step 2” and how much more the target layer surface needs to be etched, etc. In “Purge 3”, the reaction space is purged to remove non-reacted products, unwanted by-products, and any remaining etchant species by feeding the dilution gas. This etching cycle can be repeated until a desired depth of the target layer is etched.

In “Step 2”, RF power is used for generating a hydrogen plasma containing hydrogen radicals, and thus, alternatively, by using a remote plasma unit, for example, a hydrogen plasma containing hydrogen radicals can also be generated. In some embodiments, RF power is applied in pulses.

In some embodiments, the etching cycle may be conducted under the conditions shown in Table 1 below.

TABLE 1 (numbers are approximate) Conditions for deposition (PECVD as an example) Substrate temperature 0 to 200° C. (preferably 20 to 100° C.) Pressure 0.1 to 10000 Pa (preferably 1 to 1000 Pa) Noble gas (as a carrier gas and/or Ar, He dilution gas) Flow rate of carrier gas 1 to 5000 sccm (preferably 1 to 2000 sccm) (continuous) Flow rate of dilution gas 10 to 10000 sccm (preferably 50 to 5000 sccm) (continuous) Flow rate of etchant gas 1 to 1000 sccm (preferably 10 to 100 sccm); Corresponding to the flow rate of carrier gas when the etchant is vaporized using a heated bottle RF power for a 300-mm wafer 10 to 1000 W (preferably 50 to 200 W); 0.1 to 100 MHz (preferably 0.4 to 60 MHz) Duration of “RF” (Step 1) 0.1 to 10 sec. (preferably 1 to 5 sec.) Growth rate per cycle ({acute over (Å)}/cycle) 5 to 100 (preferably 40 to 80) Film thickness ({acute over (Å)}) 5 to 100 (preferably 10 to 50) Conditions for etching Substrate temperature 0 to 200° C. (preferably 20 to 100° C.) Pressure 0.1 to 10000 Pa (preferably 1 to 1000 Pa) Etching gas H₂, NH₃, H₂ + Ar/He, or NH₃ + Ar/He, Flow rate of etching gas 10 to 10000 sccm (preferably 50 to 5000 sccm) for hydrogen-containing gas; 10 to 10000 sccm (preferably 50 to 5000 sccm) for rare gas RF power for a 300-mm wafer 10 to 1000 W (preferably 50 to 200 W); 0.1 to 100 MHz (preferably 0.4 to 60 MHz) Duration of “Purge 1” 1 to 60 sec. (preferably to 10 sec., depending on chamber structure) Duration of “Step 2” 1 to 120 sec. (preferably 10 to 30 sec.) Duration of “Step 3” 1 to 60 sec. (preferably to 10 sec., depending on chamber structure) Duration of “Purge 2” 1 to 60 sec. (preferably to 10 sec., depending on chamber structure) Etching rate per cycle ({acute over (Å)}/cycle) 1 to 50 (preferably 2 to 10) Etched thickness ({acute over (Å)}) 1 to 1000 (preferably 10 to 100)

In the sequence illustrated in FIG. 2, the halogen-containing gas may be supplied using a carrier gas which can be continuously supplied to the reaction space, particularly when the halogen-containing gas is a vaporized gas of liquid material such as C₅F₈, C₅F₁₀, etc. This can be accomplished using a flow-pass switching (FPS) system (e.g., the system disclosed in U.S. patent application Ser. No. 14/829,565, filed Aug. 18, 2015, the disclosure of which is incorporated by reference in its entirety) wherein a carrier gas line is provided with a detour line having a precursor reservoir (bottle), and the main line and the detour line are switched, wherein when only a carrier gas is intended to be fed to a reaction chamber, the detour line is closed, whereas when both the carrier gas and a precursor gas are intended to be fed to the reaction chamber, the main line is closed and the carrier gas flows through the detour line and flows out from the bottle together with the precursor gas. In this way, the carrier gas can continuously flow into the reaction chamber, and can carry the precursor gas in pulses by switching the main line and the detour line.

In some embodiments, the target layer is constituted by Si alone, a metal alone, an oxide of the foregoing, or a carbonate of the foregoing, and accordingly, a suitable etchant gas (halogen-containing gas) can be selected which can form an etchant film (halogen-containing film) which in turn generates etchant species when being etched by hydrogen radicals. For example, when the target layer is constituted by SiO₂, the etchant gas suitably contains F as a halogen, whereas when the target layer is constituted by Si, the etchant gas suitably contains Cl as a halogen. A skilled artisan in the art can readily select a suitable etchant gas for the target layer in view of the present disclosure and through routine experimentation. Table 2 below shows examples. Since the etching process by the etchant species has high selectivity, it is possible to selectively etch only silicon oxide while keeping silicon nitride, for example. The etchant film may be constituted by a carbon halide which can be decomposed to generate etchant species such as a hydrogen halide by hydrogen radicals. The hydrogen radicals can be produced from an etchant gas containing hydrogen or a mixture of a hydrogen-containing gas and a rare gas.

TABLE 2 (Suitable combination) Target layer Etchant film Si, Al (a metal soluble in HCl) CCl film (by PVC, etc.) Ti, W (a metal soluble in HCl) CF film Silicon oxide CF film Silicon nitride Not etchable Metal oxide (e.g., TiO) CF film

The process cycle can be performed using any suitable apparatus including an apparatus illustrated in FIG. 1, for example. FIG. 1 is a schematic view of a plasma-assisted cyclic etching apparatus, desirably in conjunction with controls programmed to conduct the sequences described below, usable in some embodiments of the present invention. In this figure, by providing a pair of electrically conductive flat-plate electrodes 4, 2 in parallel and facing each other in the interior 11 (reaction zone) of a reaction chamber 3, applying HRF power (13.56 MHz or 27 MHz) 20 to one side, and electrically grounding the other side 12, a plasma is excited between the electrodes. A temperature regulator is provided in a lower stage 2 (the lower electrode), and a temperature of a substrate 1 placed thereon is kept constant at a given temperature. The upper electrode 4 serves as a shower plate as well, and noble gas, reactant gas, and etchant gas are introduced into the reaction chamber 3 through gas lines 21, 22, and 23, respectively, and through the shower plate 4. Additionally, in the reaction chamber 3, a circular duct 13 with an exhaust line 7 is provided, through which gas in the interior 11 of the reaction chamber 3 is exhausted. Additionally, a dilution gas is introduced into the reaction chamber 3 through a gas line 23. Further, a transfer chamber 5 disposed below the reaction chamber 3 is provided with a seal gas line 24 to introduce seal gas into the interior 11 of the reaction chamber 3 via the interior 16 (transfer zone) of the transfer chamber 5 wherein a separation plate 14 for separating the reaction zone and the transfer zone is provided (a gate valve through which a wafer is transferred into or from the transfer chamber 5 is omitted from this figure). The transfer chamber is also provided with an exhaust line 6. In some embodiments, the deposition of multi-element film and surface treatment are performed in the same reaction space, so that all the steps can continuously be conducted without exposing the substrate to air or other oxygen-containing atmosphere. In some embodiments, a remote plasma unit can be used for exciting a gas.

In some embodiments, a dual chamber reactor (two sections or compartments for processing wafers disposed closely to each other) can be used, wherein a reactant gas and a noble gas can be supplied through a shared line whereas a precursor gas is supplied through unshared lines. In some embodiments, the deposition step can be performed using an apparatus different from that for the etching step.

A skilled artisan will appreciate that the apparatus includes one or more controller(s) (not shown) programmed or otherwise configured to cause the deposition and reactor cleaning processes described elsewhere herein to be conducted. The controller(s) are communicated with the various power sources, heating systems, pumps, robotics, and gas flow controllers or valves of the reactor, as will be appreciated by the skilled artisan.

The present invention is further explained with reference to working examples below. However, the examples are not intended to limit the present invention. In the examples where conditions and/or structures are not specified, the skilled artisan in the art can readily provide such conditions and/or structures, in view of the present disclosure, as a matter of routine experimentation. Also, the numbers applied in the specific examples can be modified by a range of at least ±50% in some embodiments, and the numbers are approximate.

EXAMPLES Example 1 (Prophetic Example)

A silicon oxide film, silicon nitride film, and silicon carbide film are formed at a thickness of about 20 nm by PEALD or PECVD as a target layer on 300-mm substrates, respectively. Deposition of an etchant film (CF film) and etching of each target layer are conducted under the conditions shown in Table 3 below using the plasma-assisted etching apparatus illustrated in FIG. 1. The sequence used in each etching cycle is shown in FIG. 2.

TABLE 3 (numbers are approximate) Conditions for deposition (PECVD) Substrate temperature Room temperature Pressure 2.0 Pa Deposition gas C₂F₆ Noble gas (as a dilution gas) Ar Flow rate of deposition gas 10 sccm (etchant gas) (continuous) Flow rate of dilution gas 2000 sccm (continuous) RF power for a 300-mm wafer 100 W; 13.56 MHz (or 27 MHz, 6 MHz, or microwaves) Duration of “RF” (Step 1) 2.0 sec. Distance between electrodes 30 mm Growth rate per cycle (nm/cycle) 0.7 nm/cyc Film thickness (nm) 2.0 nm Conditions for etching Substrate temperature Same as in deposition Pressure Same as in deposition Etching gas H₂ + Ar Flow rate of etching gas H₂: 100 sccm; Ar: 1000 sccm RF power for a 300-mm wafer Same as in deposition Duration of “Purge 1”  60 sec. Duration of “Step 2”  60 sec. Duration of “Step 3”   5 sec. Duration of “Purge 2”  60 sec.

The etching cycle comprising the deposition step and the etching step is repeated 3 and 6 times for each substrate. After the etching cycles, the thickness of each target layer is measured by ellipsometry. As a result, the etching rate per cycle (EPC) of the SiO₂ film is about 0.7 nm/cycle, whereas those of the SiN film and the SiC film are nearly zero, i.e., substantially no reduction of the film thickness.

It will be understood by those of skill in the art that numerous and various modifications can be made without departing from the spirit of the present invention. Therefore, it should be clearly understood that the forms of the present invention are illustrative only and are not intended to limit the scope of the present invention. 

We claim:
 1. A method for etching a target layer on a substrate by a dry etching process which comprises at least one etching cycle, wherein an etching cycle comprises: (i) depositing a carbon halide film using reactive species on the target layer on the substrate, wherein the carbon halide film and the target layer are in contact with each other; and (ii) etching the carbon halide film using a plasma of a non-halogen hydrogen-containing etching gas without etching the target layer, which plasma alone does not substantially etch the target layer, wherein a hydrogen halide is generated as a by-product of the etching of the carbon halide film as etchant species at a boundary region of the carbon halide film and the target layer, thereby chemically etching a portion of the target layer in the boundary region without a plasma other than the plasma of the non-halogen hydrogen-containing etching gas.
 2. The method according to claim 1, wherein step (ii) continues until the carbon halide film is substantially entirely etched, indicating that substantially the entire portion of the boundary region of the target layer is etched.
 3. The method according to claim 1, wherein a duration of step (i) is correlated with a thickness of the etched portion of the target layer until the thickness of the etched portion of the target layer reaches a plateau while the duration of step (i) increases, and step (i) continues until the thickness of the etched portion of the target layer reaches the plateau or a point near the plateau.
 4. The method according to claim 1, wherein the etching cycle is repeated at least two times.
 5. The method according to claim 1, wherein step (i) continues until a thickness of the carbon halide film falls within a range of 0.5 nm to 10 nm.
 6. The method according to claim 1, wherein in step (ii), a thickness of the etched portion of the target layer is 0.1 nm to 2.0 nm.
 7. The method according to claim 1, wherein in step (i), the carbon halide film is deposited by a gas phase reaction wherein the reactive species are those of an etchant gas or gases constituted by a halogen and a carbon.
 8. The method according to claim 7, wherein the halogen is F or Cl.
 9. The method according to claim 8, wherein the halogen is F, and the etchant species is HF.
 10. The method according to claim 8, wherein the etchant gas is CxFy having a double or triple bond wherein x and y are integers and x is at least
 2. 11. The method according to claim 7, wherein the gas phase reaction is plasma-enhanced CVD.
 12. The method according to claim 11, wherein the plasma-enhanced CVD comprises: (a) continuously feeding a noble gas to a reaction space wherein the substrate is placed; (b) continuously feeding a carbon halide gas to the reaction space; and (c) after elapse of a preset duration of steps (a) and (b) without excitation of the noble gas and the carbon halide gas, applying RF power to the reaction space to deposit the carbon halide film on the target layer, wherein no oxidizing gas is fed to the reaction space throughout steps (a) through (c).
 13. The method according to claim 12, wherein a duration of step (c) is shorter than the preset duration of steps (a) and (b).
 14. The method according to claim 1, wherein in step (ii), the non-halogen hydrogen-containing etching gas is hydrogen or ammonia or a mixture of hydrogen or ammonia and a rare gas and contains no oxygen.
 15. The method according to claim 1, wherein in step (ii), the carbon halide film is etched by reactive ion etching (RIE).
 16. The method according to claim 15, wherein the RIE is capacitively-coupled plasma etching.
 17. The method according to claim 16, wherein the capacitively-coupled plasma etching comprises: (a) continuously feeding a reactant gas to a reaction space wherein the substrate is placed; and (b) after elapse of a preset duration of step (a) without excitation of the reactant gas, applying RF power to the reaction space to etch the carbon halide film and the target layer.
 18. The method according to claim 1, wherein the target layer is constituted by SiO₂, metal oxide, or a metal, which is etchable by a hydrogen halide.
 19. The method according to claim 1, wherein step (i) and step (ii) are continuously conducted in the same reaction chamber. 